1. Field of the Invention
The present invention relates to the preparation of aromatic compounds bearing at least two isocyanate substituents, and, more especially, to the preparation of toluene diisocyanate and isomers thereof, whether alone or in admixture.
2. Description of the Prior Art
The preparation of aromatic compounds bearing one or more isocyanate substituents by reacting amines with phosgene in the gaseous phase has long been known to this art, albeit such reaction has essentially been limited to the conversion of monofunctional amines.
Thus, the gas phase phosgenation of aliphatic monoamines such as methylamine or ethylamine, or of aromatic monoamines such as aniline, is described in GB-656,726 (Monsanto). However, the reaction of polyfunctional amines with phosgene is not described.
GB-1,165,831 (ICI) describes the gas phase reaction of aromatic or aliphatic mono- or polyfunctional amines with phosgene with a view to preparing the corresponding isocyanates, such reaction being carried out in a cylindrical reactor fitted with a mechanical stirrer. This process presents disadvantages by reason of the use of rotating components of the mechanical stirrer, in particular problems of sealing at the rotating shaft and problems of fouling and therefore of blocking the stirrer as a consequence of the formation of adhesive reaction byproducts.